This application relates to techniques, devices, and systems for controlling and compensating for optical dispersions in optical media such as optic fiber links in various systems including wavelength-division multiplexed (WDM) optical systems.
Many optical fibers and other optical transmission media can exhibit chromatic dispersion where different spectral components at different wavelengths in an optical signal travel at different speeds. One parameter for characterizing the dispersion is the group velocity which is related to the derivative of the propagation constant of an optical wave with respect to frequency. The first-order group velocity dispersion is typically expressed as a change in light propagation time over a unit length of fiber with respect to a change in light wavelength. An optical pulse comprised of different optical spectral components, therefore, can be broadened or distorted in shape after propagation through a distance in such a dispersive optical medium. This chromatic dispersion effect can be undesirable and even adverse for certain applications such as optical communication systems where information is encoded, processed, and transmitted through optical pulses. The pulse broadening caused by the dispersion can limit the transmission bit rate, the transmission bandwidth, and other performance factors of the optical communication systems.
In addition to chromatic dispersion, some optical transmission media including optical fibers may be optically birefringent to cause polarization-mode dispersion (“PMD”) in which the media exhibit different refractive indices for light with different polarizations along two orthogonal principal directions. Therefore, an optical signal, that comprises two polarization components along the two orthogonal principal directions for each frequency, can be distorted after propagation through the transmission medium since the two components propagate in different group velocities. This polarization-mode dispersion is generally independent of the chromatic dispersion and may cause signal distortions even when the chromatic dispersion is fully or substantially compensated, or is sufficiently small to cause pronounced effects.
The degree of PMD may be approximately characterized by the average differential group delay (“DGD”) between two principal states of polarization. Typical causes for such birefringence in fibers include, among others, imperfect circular core and unbalanced stress in a fiber along different transverse directions. The axis of birefringence of the optical fiber may change randomly depending on the external conditions. Thus, the DGD in an actual PMD fiber is not a fixed value but a random variable that generally has a Maxwellian probability density function.
One way to mitigate dispersion effects in dispersive optical fibers and other optical transmission media is dispersion compensation by introducing dispersions in an optical signal to negate the dispersions accumulated in that optical signal. Dispersion devices may be used to add artificially-controlled dispersion to the dispersion in the optical signal caused by the transmission medium to modify or control the total dispersion in an optical signal. In dispersion compensation applications, for example, a dispersion device may be designed to produce dispersion that substantially cancels the dispersion caused by the transmission medium. At a given location in an optical link, however, the dispersion in an optical signal may change over time due to factors such as fluctuations in the dispersion caused by variations in temperature or stress in a given optical path of the signal and changes in the physical path of the signal due to switching or routing operations of the nodes. Therefore, it may be desirable to dynamically tune such dispersion compensation or control in response to those and other changes in the dispersion.
Notably, in WDM systems, multiple WDM optical channels at different wavelengths are simultaneously transmitted through a single fiber. Since the dispersions in different WDM channels may be different, it may be desirable to provide different amounts of dispersion compensation to different WDM channels at the same time. In addition, since the dispersion in the WDM channels may vary over time, it may also be desirable to adjust the dispersion compensation in time for different WDM channels.